feat: rename and export LegacyAlfrescoApi in public-api

[mserwicki]

Please check if the PR fulfills these requirements

[ ] The commit message follows our [guidelines](https://github.com/Alfresco/alfresco-js-api/wiki/Commit-format)
[ ] Tests for the changes have been added (for bug fixes / features)
[ ] Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] Documentation
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

We hide LegacyAlfrescoApi helper class, that is needed in public api's to provide custom AlfrescoApiType for ApiClient class

What is the new behavior?

Now the LegacyAlfrescoApi is exposed in pubic, so users can create APis using AlfrescoApiType not only HttpClient

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

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