wip: "Loaded" and "Partial" Periods

[peaBerberian]

This is a big work in progress that includes many PR done here (at least #702, #701, #700 and #694) - though all those other PRs can already be merged.

This feature allows two things:

• only fetch MetaPlaylist sub-contents as we need it
• implement DASH xlinks with an actuate attributes set at onResolve

More simply put, it allows to download temporal sub-parts of a content only when we need them (and only if there is an external link to download them).

This PR works for now with MetaPlaylist sub-contents, but there is still some work to do:

• [ ] make it work with DASH xlinks
• [ ] limit to a single Period load at a time (for now, when multiple buffers ask to load the next Period, we will be left with as much requests which is not necessary)
• [ ] update regular manifest update logic and test it
• [ ] add some unit tests to src/manifest/__tests__/manifest.test.ts and src/peroid/__tests__/period.test.ts
• [ ] resolve every part of the code prefixed with an XXX TODO comment