[pkg/ottl] Add string access for span kind

[TylerHelmuth]

Description:
Adds a new access available to ottlspan and ottlspanevent that allows getting and setting the span kind using a string. This accessor is necessary to facilitate #21054, as the current internal/filterspan logic allows filtering span kind based on the string interpretation.

Testing:
Added new tests

Documentation:
Updated ottlspan README with new accessor.

[TylerHelmuth]

/cc @dmitryax bc I had to add a new function to traceutil and it looks like we want to get rid of those eventually.

[jiekun]

cmiiw OTTL user (like tail sampling processor) will benefit from all those new OTTL feature right? I'm trying to be understand that what kind of change will benefit OTTL user component, and what will not :/

Edit: OK so the core idea is to secure that I am writing the correct code for tail sampling processor, allowing it to get the latest update of OTTL (everytime, in the future) :)

[TylerHelmuth]

@jiekun A change like this would be immediately useable by any component that uses OTTL's standard contexts (in Contrib all the components using OTTL use the standard contexts) so the tailsamplingprocessor would get it.

That is in contrast to new functions which a component needs to opt into.

[evan-bradley]

Looks good to me.