[r] Use cached SOMA context rather than re-creating

[mojaveazure]

Cache a SOMA context during $new() as a private field when instantiating new objects. This cached context can then be used in other methods. Note, this PR is not yet changing the UX of config/context options. As the context from soma_context() is cached globally, we can modify the global one before instantiating to test customized cached contexts

Modified SOMA methods:

• TileDBObject$new(): creates context and caches it within the object
• TileDBGroup$create(): no longer caches context, instead uses cached context from $new()
• TileDBGroup$open(): uses cached context rather than calling soma_context()

SC-55084

[eddelbuettel]

Good additions in the last comit.