[file_server] Make suffix type mappings case-insensitive

[ravenblackx]

Commit Message: [file_server] Make suffix type mappings case-insensitive
Additional Description: Very small change to file_server, after realizing that we have an environment that serves both PNG and png files. It doesn't make much sense to have to define every combination of case for suffixes (maybe there's some Png or pnG), so making it case-insensitive makes sense.

One way to do that would be to accept any case and convert at init time - but that would provoke a confusing behavior if someone configured {"png": "image/png", "PNG": "application/octet-stream"}. We could do a deduplication validation after the fact to prevent that, but that adds a bunch of extra code for something that's trivially simple if we just enforce that it be configured in lowercase only at the proto validation level (which should be fine as a change since the filter is one day old and marked WIP).

A bonus effect of doing it this way is that protovalidate is also validating "no periods" which I previously validated in code, so that code and its test can be removed.
Risk Level: Negligible, minor change to brand new filter.
Testing: Some removed. Manually validated that the protovalidate rule is functioning as expected.
Docs Changes: n/a
Release Notes: Added previously-forgotten changelog.
Platform Specific Features: n/a

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @wbpcode
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #43668 was opened by ravenblackx.

see: more, trace.

[phlax]

lgtm, thanks @ravenblackx