[pkg/ottl] Add math operators to the grammar

[TylerHelmuth]

Is your feature request related to a problem? Please describe.

Currently the grammar does not have a solution for combining numeric values.

Describe the solution you'd like

Update the grammar to understand +, -, *, and /. For now this symbols will only apply to int64 and float64, but could be made to work on other types that the grammar understands later.

Describe alternatives you've considered

Add functions to represent Add, Sub, Mult, and Div. While possible to solve this problem with new functions, updating the grammar to understand these operators natively will mean less overall functions to maintain and more flexibility with combining non-numeric types and mixed types.

Additional context

The specific use case I have heard from an OTEL user that will benefit from this feature is being able to calculate the difference between a span's end_time_unix_nano and start_time_unix_nano with a condition.