Add Mcap Appender to the Typescript Core package

[alexern14]

Public-Facing Changes

• Added data end offset information to Typescipt Mcap Indexed Reader.
• Added an Mcap Appender to the Typescript Mcap core package.

Description

• Added data end offset information to Typescipt Mcap Indexed Reader for Mcap appending purposes.
• Added an Mcap Appender to the Typescript Mcap core package in order to append metadata and attachments to existing Mcap files via the @mcap/core package.

[jtbandes]

Thanks for the PR, this is interesting functionality. Can I ask why you chose to make this a separate class and not a feature of the existing writer class? (e.g. "open in append mode")

[alexern14]

@jtbandes I originally started this by just changing the functionality of the writer class to be able to append, and while doing some cleanup I just created a separate class and was not quite sure how you guys would want this organized. I am open and willing to changing it to be additional functionality on the writer class though!

[jtbandes]

I'm not sure how easily it would slot into the existing class but I do think, from the standpoint of a user, it would be more natural to be able to use the same writer class for either creating a new file or modifying an existing one. I think it could be worth exploring this (and maybe opening a separate PR if you think it's not clear which is better).

I also think the user would rather not have to do things like setStatistics / addChunkIndex explicitly, but instead the writer/appender should populate itself by reading the file during creation. This might necessitate an async create function, like we have for the indexed reader:

mcap/typescript/core/src/McapIndexedReader.ts

Line 78 in f87352f

static async Initialize({

[alexern14]

@jtbandes Thanks for the feedback! I just put up a different PR with that proposed implementation here #1016