Related to T2105

Related to T2105

Related to T2105

Add missing object_type class attributes on MetadataAuthority, MetadataFetcher, and RawExtrinsicMetadata.

It can help in pytest's diffs

to fix a deprecationg warning.

to prevent warnings

D3560

 - Use dia directly to convert from .dia to .svg (inkscape would use dia via a
 plugin anyway)
 - Add proper runes to detect inkscape >= 1 and use the export options for that.

As NamedTuple inherits from tuple, msgpack serializes it like a tuple,
which makes it indistinguishable from a tuple when deserializing,
which is an issue for the RPC API.

It allows easy conversion of Union[ImmutableDict, Dict] to ImmutableDict.

So they are truly immutable now.

This will allow swh-storage to have a signature for *_metadata_add
that is consistent with other *_add endpoints.

Add a new extra_headers attribute on Revision and use it for computing
the revision's id instead of extract it from the metadata field.

Only accept (bytes, bytes) as extra_header.

Add a post init hook to Revision to initialize this new attribute from
given metadata, if any, for bw compat.

Also amend the revision_d hyptothesis strategy to generate extra_headers.

When Software Heritage persistent identifiers were introduced, they were not
yet abbreviated as SWHIDs.

Now that abbreviation is growing adoption, rename some functions and types in
swh.model.identifiers for consistency:

  - PersistentId -> SWHID

  - persistent_identifier -> swhid

  - parse_persistent_identifier -> parse_swhid

Backward compatibility with previous naming is maintained but deprecation
warnings are introduced to encourage the use of the new names.

Numerous variables in swh.model codebase have also been renamed accordingly.

Also rework and improve documentation.

Related to T2310

this aims at preventing constant usage of isinstance() based dispatch
code when writing generic code handling model entities.

For example, the "object_type" argument of JournalWriter.write_addition() has
become superflous now we only pass model entities, etc.

This idea comes olasd's reading of mypy doc:

  https://mypy.readthedocs.io/en/latest/literal_types.html#tagged-unions

This comes with a refactoring of from_dict.DiskBackedContent to make
it *not* inherit from model.Content: object_type being Final, it cannot
be overloaded.

Related to T2310

Related to T2310