NIP-14: subject tag support for text events

[a-khushal]

Is your feature request related to a problem? Please describe.
some clients use NIP-14 subject tags in text notes, but nostream does not support NIP-14 yet.

Describe the solution you'd like
add support for NIP-14 subject tags, add tests for it, and also update docs of supported NIPs.

Additional context

• https://github.com/nostr-protocol/nips/blob/master/14.md
• https://nips.nostr.com/14

[a-khushal]

@cameri can i work on this?

[a-khushal]

plan of implementation:

1. accepting ["subject", "<string>"] tag on text note events (kind: 1) in normal relay flow
2. Preserving the subject tag end-to-end (ingest -> store -> read/relay output), withhout any kind of mutation.
3. add NIP-14 to supported NIPs

[a-khushal]

@cameri tested nip-14 locally, subject tag is already coming back from relay as a generic tag on main (screenshot attached). so don’t think there’s any behavior change needed here

we can still keep this as a non-behavior change with constants, tests and docs to match the NIPs, but do we actually need it?