Create IPIP: UnixFS and Gateway support for explicit MIME/Content-Type

This is a placeholder issue for creating IPIP to add support for storing explicit MIME/Content-Type in UnixFS DAG itself, like we already do for opt-in `mode` and `mtime`, and acting on its presence on Gateways.

There is alrernative approach to allow stating explicit content-type header via `_headers` file (similar to recently shipped `_redirects`) – some notes in https://github.com/ipfs/specs/issues/257

## Context

UnixFSv2 never happened, but ability to explicitly specify `Content-Type` was one of my asks: https://github.com/ipld/legacy-unixfs-v2/issues/11

Years later, we still guess `Content-Type` on gateways.
While it is fine for most of the time, but we should provide users with ability to explicitly set media type at the time of data onboarding.

Since 2018 we made some related development: we've added opt-in support for `mode` and `mtime` attributes. ~2020 (https://github.com/ipfs/specs/issues/217#issuecomment-582369839). Support for `mode` and `mtime` was implemented in JS-IPFS a while ago, Kubo still has PR open (https://github.com/ipfs/go-unixfs/pull/117).

Initial idea (details to be fleshed out in IPIP) is to introduce optional `mtype` similar way.
This is alternative to introducing `_headers` file mentioned in https://github.com/ipfs/specs/issues/257.

Ref.
- old discussions https://github.com/ipfs/kubo/issues/2164
- Media Type registry at IANA: https://www.iana.org/assignments/media-types/media-types.xhtml
- IETF RFC about Media Types https://www.rfc-editor.org/rfc/rfc2046.html
- Some examples how the same CID can be returned with different `Content-Type`: https://github.com/ipfs/in-web-browsers/issues/152
- Kubo Gateway guessing `Content-Type` based on filename and magic bytes: https://github.com/ipfs/kubo/blob/v0.17.0-rc2/core/corehttp/gateway_handler_unixfs_file.go#L55-L87
- Gateway support for overriding filename: https://github.com/ipfs/specs/blob/main/http-gateways/PATH_GATEWAY.md#filename-request-query-parameter
- IPFS users leverage this for adjusting Content-Type when `/ipfs/cid` is requested without any filename, but with built-in support for explicit content type we could make it "just work".

## TODO

- [ ] wait until UnixFS specs land (#331)
- [ ] create IPIP against UnixFS specs that
- [ ] defines canonical way of storing explicit MIME (content/media type) in UnixFS root dag-pb blocks.
- [ ] modifies Gateway spec to disable mime-sniffing and use sanitized (ASCII-only) value from dag-pb in `Content-Type` header
- [ ] Create reference implementation in Kubo
- [ ] adds optional `--content-type` to `ipfs add`
- [ ] skip sniffing and set `Content-Type` header on gateway, if present