Add deprecations for 'legacy' extension system

[braingram]

The 'legacy' extensions should raise deprecation warnings in 2.15 as they will be removed in 3.0. Warnings should be issued for:

• CustomType: add deprecation to CustomType #1359
• Resolver (see comment below): deprecate asdf.resolver submodule #1362
• asdf.types (this includes format_tag) deprecate asdf.types #1401
• asdf.tests.helpers.assert_extension_correctness: Deprecate/assert extension correctness #1388
• asdf.extension.get_default_resolver: deprecation asdf.extension legacy functions #1409
• asdf.extension.get_cached_asdf_extension_list: deprecation asdf.extension legacy functions #1409
• asdf.extension.default_extensions: deprecation asdf.extension legacy functions #1409
• asdf.type_index: deprecate type_index #1403
• asdf.AsdfFile.run_hook and similar: deprecate AsdfFile attrs and methods that use legacy extension api #1417
• asdf.types.AsdfType: deprecate asdf.types #1401
• asdf.types.ExtensionType: deprecate asdf.types #1401
• asdf.extension.AsdfExtension: Deprecate AsdfExtension, AsdfExtensionList, BuiltinExtension #1429
• asdf.extension.AsdfExtensionList: Deprecate AsdfExtension, AsdfExtensionList, BuiltinExtension #1429
• asdf.extension.BuiltInExtension: Deprecate AsdfExtension, AsdfExtensionList, BuiltinExtension #1429
• asdf_extensions entry point: deprecate asdf_extensions entry point #1361

[braingram]

• Resolver
  The documentation mentions a deprecated resolver mechanism:
  

  asdf/docs/asdf/developer_overview.rst

  Lines 96 to 97 in cbc066d

  	reasons explained later. The resolver system has been deprecated in favor
  	of resource mappings; new code should use the latter instead.

  
  Some of this has deprecation warnings.
  

  asdf/asdf/extension/_legacy.py

  Lines 132 to 139 in cbc066d

  	@property
  	def tag_to_schema_resolver(self):
  	"""Deprecated. Use `tag_mapping` instead"""
  	warnings.warn(
  	"The 'tag_to_schema_resolver' property is deprecated. Use 'tag_mapping' instead.",
  	AsdfDeprecationWarning,
  	)
  	return self._tag_mapping

  
  Some does not:
  

  asdf/asdf/extension/_legacy.py

  Lines 253 to 257 in cbc066d

  	def get_default_resolver():
  	"""
  	Get the resolver that includes mappings from all installed extensions.
  	"""
  	return default_extensions.resolver

I'm not familiar with the resolver. @WilliamJamieson do you think we'll still need this in 3.0? If it's uncertain should we add a deprecation with the hope we can remove it?

[eslavich]

The resolver is what the old-style extensions use to locate schema content for a URI. If we're removing CustomType and friends then we won't need it.

[eslavich]

A prerequisite to removing the above is translating everything under asdf.core.tags to Converter and providing some new entity class for each tagged object. Probably the user should see some kind of deprecation warning when they import from those modules? But we'll need to prevent the warning when asdf itself imports them. Maybe we move their contents to an underscored module that asdf can import from, and create aliases in the original module names with a warning?