Implement context load, context save, and context reset

[eliaskosunen]

Closes https://github.com/tenzir/issues/issues/1523, companion PR: https://github.com/tenzir/tenzir-plugins/pull/175

• context update was previously overloaded on its input type. Now it only accepts events. The bytes-overload is removed (it wasn't implemented by contexts), and the void-overload is called context reset.
• context load and context save are added, that load from and serialize to bytes, respectively.
  • They are transformations on bytes<->events, and don't perform I/O themselves: the most common use case is probably that load <serialized-binary-file> is piped to context load, and context save is piped to save <serialized-binary-file>. A carbon-copy of a context can be created with context save foo | context load bar.
  • The contexts already implemented this functionality, the necessary plumbing to call them just wasn't there, yet

As a table:

Before	After	Description
<events> | context update <ctx>	(unchanged)	Update context with events
<bytes> | context update <ctx>	(removed, wasn't implemented)	Update context with bytes
context update <ctx> [flags]	context reset <ctx> [flags]	Reset context state
N/A	context save | <bytes>	Serialize context save into bytes
N/A	<bytes> | context load	Load context state from bytes

[Dakostu]

Left some comments.

[Dakostu]

I preemptively approve now, with some small suggestions remaining.

[Dakostu]

@eliaskosunen Please also add a changelog item regarding the new subcommands.