refactor(sdk-trace-base)!: remove Span class from exports

[david-luna]

Which problem is this PR solving?

Removes the Span constructor from the exports so users are compelled to use tracer.startSpan API which is safer. Instead of the class the SDK exports now a type with the same properties.

Ref: #3597

Short description of the changes

• make Span a type and not a class
• rename span class to SpanImpl
• use the SpanImpl constructor internally (src & tests)
• export only the Span type in index.ts

Type of change

Please delete options that are not relevant.

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• compiled all packages
• ran tests for all packages

Checklist:

• merge main branch into next ([next] merge changes from main #5052)
• Followed the style guidelines of this project