* feat: replace hardcoded span start function names with regex patterns

Replaces the hardcoded list of function names and packages with a list
of regex patterns that may be extended using the
`-extra-start-span-signatures` command line option.

The format is `<regex>:<telemetry-type>`. Ideally this would just be a
list of regex patterns, but we need to know what telemetry library we're
matching so we don't run RecordError on opencensus.

* tests: update tests to use new default config

The tests need to unconditionally include the
DefaultStartSpanSignatures, which was not possible with the old harness.

Instead of returning an object directly, they now return a function to
prepare a config, which ensures they all start out with the default
config as a base.

* Add test, skip validating spans within starter funcs

* Hack: fix check for start func method name

* fix: increase robustness of skipping custom matchers

Instead of checking if the function name matches, which might result in
overzealous exclusions, prefer to match against the function signature
itself

* docs: document new behaviour

* Remove sig split in config

* nit: small wording changes

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Co-authored-by: josh <joshua.timmons1@gmail.com>