Allow blob num_context_lines to be specified

What does this MR do and why?

Adds an optional param num_context_lines to allow users to specify how many lines of code their blob search results should show before and after the matching line, defaulting to 2.

This param is available for web and API.

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	Web	API
default
num_context_lines=10

How to set up and validate locally

Verify that the number of code lines before and after a matching search term changes when specifying num_context_lines:

1. Run a blob search web request without num_context_lines: http://127.0.0.1:3000/search?scope=blobs&search=test&search_type=advanced
2. Run a blob search web request with num_context_lines=10: http://127.0.0.1:3000/search?scope=blobs&search=test&search_type=advanced&num_context_lines=10
3. Run a blob search API request without num_context_lines: http://127.0.0.1:3000/api/v4/search?scope=blobs&search=test&search_type=advanced
4. Run a blob search API request with num_context_lines=10: http://127.0.0.1:3000/api/v4/search?scope=blobs&search=test&search_type=advanced&num_context_lines=10

Related to #468311 (closed)