elasticapm: introduce StartTransactionOptions

[axw]

Introduce Tracer.StartTransactionOptions, a variant
of Tracer.StartTransaction which accepts a third
parameter of type TransactionOptions. This replaces
the TransactionOption functional options; StartTransaction
no longer accepts variadic arguments.

The purpose of this is to avoid allocations due to
closures in hot code paths, such as starting transactions
in http middleware, where setting trace context will
be commonplace.

[codecov-io]

Codecov Report

Merging #165 into master will decrease coverage by 2.81%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
- Coverage   76.95%   74.13%   -2.82%     
==========================================
  Files          72       81       +9     
  Lines        4148     4934     +786     
==========================================
+ Hits         3192     3658     +466     
- Misses        730     1097     +367     
+ Partials      226      179      -47

Impacted Files	Coverage Δ
transaction.go	93.58% <100%> (+12.33%)	⬆️
module/apmhttp/handler.go	72.5% <100%> (+4.16%)	⬆️
internal/apmdebug/debug.go	20% <0%> (ø)
internal/fastjson/marshaler.go	56.06% <0%> (ø)
internal/pretty/formatter.go	0% <0%> (ø)
internal/radix/radix.go	39% <0%> (ø)
internal/krtext/indent.go	0% <0%> (ø)
internal/fastjson/writer.go	66.23% <0%> (ø)
internal/pretty/zero.go	0% <0%> (ø)
internal/pretty/pretty.go	0% <0%> (ø)
... and 21 more

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