client: Accept MIME parameters in content types

[caniko]

ContentType currently accepts only the two bare media types. That rejects valid Secret Service values with MIME parameters: the specification itself uses text/plain; charset=utf8, and go-keyring sends that exact value.

This keeps the existing Text/Blob model, but compares only the base media type. Matching is trimmed and case-insensitive, outgoing serialization stays unchanged, and unknown base types still report the original full value.

I hit this through kubelogin: writing its token cache failed before kubectl could return cluster credentials.

Tested with:

• the full client test suite with native crypto
• the full client test suite with OpenSSL crypto
• a go-keyring Set/Get/Delete round trip against oo7

[caniko]

Tested on my own NixOS flake deployed with Cosmic DE

[bilelmoussaoui]

thank you!