Add Debian API endpoint for {In,}Release files

What does this MR do?

This is item 3.3.3 of &6057 (comment 582697034).

Database

This query was reviewed when the finder was first implemented in this MR, so we do not need an additional database review here.

Here is the query for reference

SELECT "packages_debian_project_distributions".* 
FROM "packages_debian_project_distributions" 
WHERE "packages_debian_project_distributions"."project_id" = 1
AND (
  "packages_debian_project_distributions"."codename" = 'foo' 
  OR 
  "packages_debian_project_distributions"."suite" = 'foo'
) 
ORDER BY "packages_debian_project_distributions"."id" DESC 
LIMIT 1;

Limit  (cost=3.18..3.18 rows=1 width=368) (actual time=0.033..0.033 rows=0 loops=1)
   Buffers: shared hit=8
   I/O Timings: read=0.000 write=0.000
   ->  Sort  (cost=3.18..3.18 rows=1 width=368) (actual time=0.031..0.032 rows=0 loops=1)
         Sort Key: packages_debian_project_distributions.id DESC
         Sort Method: quicksort  Memory: 25kB
         Buffers: shared hit=8
         I/O Timings: read=0.000 write=0.000
         ->  Index Scan using uniq_pkgs_debian_project_distributions_project_id_and_suite on public.packages_debian_project_distributions  (cost=0.14..3.17 rows=1 width=368) (actual time=0.009..0.010 rows=0 loops=1)
               Index Cond: (packages_debian_project_distributions.project_id = 1)
               Filter: ((packages_debian_project_distributions.codename = 'foo'::text) OR (packages_debian_project_distributions.suite = 'foo'::text))
               Rows Removed by Filter: 0
               Buffers: shared hit=5
               I/O Timings: read=0.000 write=0.000

Time: 0.548 ms  
  - planning: 0.480 ms  
  - execution: 0.068 ms  
    - I/O read: 0.000 ms  
    - I/O write: 0.000 ms  
Shared buffers:

hits: 8 (~64.00 KiB) from the buffer pool
reads: 0 from the OS file cache, including disk I/O
dirtied: 0
writes: 0

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team