move pointer tag common utility functions to xml_utils and add pointer tag support in HTML XBlock

[ttqureshi]

• resolves: Add LegacyXmlMixin in xml_utils and support the pointer tag format in the HTML XBlock. public-engineering#435

The utility functions in xml_utils are derived from the XmlMixin class in the edx-platform. We are only retaining the functionality related to pointer tag parsing, ensuring that Extracted XBlocks can be exported and imported in the pointer tag format without any issue.

Here are the detailed references:

Utility Functions	References
name_to_pathname	https://github.com/openedx/edx-platform/blob/master/xmodule/xml_block.py#L22
is_pointer_tag	https://github.com/openedx/edx-platform/blob/master/xmodule/xml_block.py#L30
serialize_field	https://github.com/openedx/edx-platform/blob/master/xmodule/xml_block.py#L54
deserialize_field	https://github.com/openedx/edx-platform/blob/master/xmodule/xml_block.py#L71
own_metadata	https://github.com/openedx/edx-platform/blob/master/xmodule/modulestore/inheritance.py#L335
apply_pointer_attributes	https://github.com/openedx/edx-platform/blob/master/xmodule/xml_block.py#L499-L506
format_filepath	https://github.com/openedx/edx-platform/blob/master/xmodule/xml_block.py#L422
file_to_xml	https://github.com/openedx/edx-platform/blob/master/xmodule/xml_block.py#L178
load_file	https://github.com/openedx/edx-platform/blob/master/xmodule/xml_block.py#L188
load_definition_xml	https://github.com/openedx/edx-platform/blob/master/xmodule/xml_block.py#L412
EdxJSONEncoder	https://github.com/openedx/edx-platform/blob/master/openedx/core/lib/json_utils.py

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[farhan]

Seems good, just minor doc improvements

[farhan]

Good work ⭐

[farhan]

@ttqureshi Please mention, how did you do its manual testing

[ttqureshi]

TESTING NOTES:
I created a course that contained various HTML blocks (Text, IFrame, etc.). Then, I exported the course to verify that the definitions were being exported in the expected pointer tag format. To confirm that the HTML pointer tags were generated correctly, I compared my course export with the export of the built-in HTML block—both manually and using this script.

After that, I imported the course (which included the pointer tag definitions of the HTML block), and all HTML blocks were initialized correctly, as expected.

[ttqureshi]

@kdmccormick this PR is up for your reivew. Please take a look.

[kdmccormick]

Could you please add type annotations for every argument and return value?

Otherwise, looks good!

[ttqureshi]

PR is up for the final pass.