Draft: Cache composer version page SHA

What does this MR do?

Adds an initial caching skeleton for composer.

This MR introduces saving the SHA checksum of the inner index pages for composer. The checksum is saved together with the package metadata. Notice that the cached value is saved here but yet not used.

Future work:

• Use the cached checksum to build in "mid layer" index pages.
• Introduce caching invalidation
• Cache the "mid layer" checksums

Migration

== 20201209193551 AddVersionShaCacheToComposerMetadata: reverting =============
-- remove_column(:packages_composer_metadata, :version_cache_sha, :binary)
   -> 0.0011s
== 20201209193551 AddVersionShaCacheToComposerMetadata: reverted (0.0051s) ====

== 20201209193551 AddVersionShaCacheToComposerMetadata: migrating =============
-- add_column(:packages_composer_metadata, :version_cache_sha, :binary, {:null=>true})
   -> 0.0012s
== 20201209193551 AddVersionShaCacheToComposerMetadata: migrated (0.0059s) ====

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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

Related to #290288 (closed)