Support .textproto extension for output files in Protobuf Text Format

[smolkaj]

The official file extension for Protobuf Text Format files is .textproto: https://protobuf.dev/reference/protobuf/textformat-spec/#text-format-files

But IIUC, currently p4c requires .txt instead in a bunch of places, e.g. to generate P4infos or table entry files.

Today:

    p4test --arch v1model \
        --p4runtime-files p4info.txt \
        --p4runtime-entries-files entries.txt \
        table-entries-ternary-bmv2.p4

Proposed:

    p4test --arch v1model \
        --p4runtime-files p4info.textproto \
        --p4runtime-entries-files entries.textproto \
        table-entries-ternary-bmv2.p4

The proposal is to start supporting .textproto and, optionally, to slowly phase out support for .txt, e.g. by initially giving a warning and eventually failing with an error.

@jafingerhut for visibility, as this came up in my review of p4lang/p4runtime#432.

[qobilidop]

Just one minor update. It seems the official recommendation has become .txtpb (from the same source: https://protobuf.dev/reference/protobuf/textformat-spec/#text-format-files).

[smolkaj]

Thanks for the update!

(I do wonder how many times more we will decide to change this recommendation.....)