Lowercase object metadata keys before storing

[zubron]

lakeFS currently stores object metadata keys with title-case (e.g., Foo, MyKey), but S3 stores them in lowercase per the S3 specification:

Amazon S3 stores user-defined metadata keys in lowercase

Why this happens:
Go's HTTP library canonicalizes HTTP headers to title-case (e.g., X-Amz-Meta-Foo) which is correct HTTP behaviour. However, the metadata key (the part after x-amz-meta-) should be extracted and lowercased before storage, since it's data, not an HTTP header name.

Current behaviour:

HTTP Request: X-Amz-Meta-Foo: bar
lakeFS stores: Foo → bar          (title-cased)
S3 stores:     foo → bar          (lowercased)

Impact:

• Client code expecting metadata['foo'] fails to find metadata['Foo']
• Breaks S3 compatibility

[arielshaqed]

CC @talSofer. This affects all object metadata obviously, part of misalignment between AWS S3 and lakeFS object metadata.

[talSofer]

Thanks @arielshaqed. While fixing this issues I suggest documenting S3-compatibility in https://docs.lakefs.io/latest/understand/glossary/#object-metadata

As for metadata search queries, the user impact depends on the client, so let's add a note to https://docs.lakefs.io/latest/datamanagment/metadata-search/#how-to-search-by-metadata with a reminder on how user metadata is saved in lakeFS and its implications on queries.