ref: Add span options aliases (WithSpanSampled, WithOpName, WithTransactionName)

[tonyo]

As a follow-up to #611, let's unify the (applicable) span options by prefixing them with "With":

SpanSampled -> WithSpanSampled
OpName -> WithOpName
TransactionName -> WithTransactionName

The original names are deprecated: not removed right away, but there's a task to do it in a version or two: #625

Not touching ContinueFromRequest, ContinueFromHeaders, and ContinueFromTrace since they have slightly different semantics.

Why?

1. Without the prefix some newly introduced span option names collide with existing types names and/or Span fields.
2. This is a common pattern e.g. in OpenTelemetry: https://github.com/open-telemetry/opentelemetry-go/blob/1b55281859bfaf4b03b7968a059c5239cf8c7a20/trace/config.go#L278-L333

Note: some docs will have to be updated after this change is released.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.08 🎉

Comparison is base (e70db81) 79.87% compared to head (57fb4d2) 79.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #624      +/-   ##
==========================================
+ Coverage   79.87%   79.95%   +0.08%     
==========================================
  Files          38       38              
  Lines        3875     3881       +6     
==========================================
+ Hits         3095     3103       +8     
+ Misses        673      671       -2     
  Partials      107      107              

Impacted Files	Coverage Δ
http/sentryhttp.go	87.27% <100.00%> (ø)
otel/span_processor.go	94.33% <100.00%> (ø)
tracing.go	89.25% <100.00%> (+0.55%)	⬆️

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