Extending Lhotse dataloading to text/multimodal data

[pzelasko]

This PR adds a very basic support for incorporating text-only data into Lhotse samplers to enable text and multimodal dataloading. Highlights:

• new ABC SamplingConstraint that generalizes TimeConstraint, and allows to create other types of constraints to decide when to stop sampling a mini-batch as well as how to determine the "size" of an example (e.g. for audio its duration, but for text it may be sth like num tokens)
• dynamic samplers have a new argument called constraint where SamplingConstraint instances may be passed directly
• TokenConstraint which is almost identical to TimeConstraint but uses num_tokens / max_tokens
• very basic dataclass TextExample that wraps text/tokens, CutSet can be used to yield those (just pass text iterator to CutSet like CutSet(text_example_iter)) (it's not super clean but it works; trying to figure out if we can make this cleaner)
• unit tests illustrating how to use this for text dataloading and even for mixed modality dataloading (text and audio data together in a mini-batch)

This is stretching the original scope of Lhotse a bit, but I feel like it's worth it: we accumulated a bunch of solid techniques here and it'd be a pity to have to use something completely different for multimodal modeling, especially when so little changes are required to make it work here. Would love to know your thoughts @danpovey @csukuangfj @desh2608 @m-wiesner

[pzelasko]

Note that this is an experimental feature: let us know if you're running into issues with this.