Add support for Event#getSequenceNumber (logback 1.3+)

[brenuart]

closes #830

The existing SequenceJsonProvider is enhanced to adapt its behaviour depending on which version of Logback is found on the class path:

• Logback >= 1.3: get the sequence value from the event itself as long as a SequenceNumberGenerator is defined in the LoggerContext. If not, revert to using a locally incremented sequence number (previous behaviour)
• Logback < 1.3: use a locally incremented sequence number

[codecov]

Codecov Report

Merging #843 (8e45af7) into main (c126999) will decrease coverage by 0.03%.
The diff coverage is 75.75%.

@@             Coverage Diff              @@
##               main     #843      +/-   ##
============================================
- Coverage     70.84%   70.80%   -0.04%     
- Complexity     1261     1270       +9     
============================================
  Files           168      170       +2     
  Lines          4829     4854      +25     
  Branches        505      507       +2     
============================================
+ Hits           3421     3437      +16     
- Misses         1155     1161       +6     
- Partials        253      256       +3     

Impacted Files	Coverage Δ
.../net/logstash/logback/composite/JsonProviders.java	90.47% <ø> (-0.44%)	⬇️
...gstash/logback/composite/SequenceJsonProvider.java	100.00% <ø> (ø)
...omposite/accessevent/AccessEventJsonProviders.java	67.56% <0.00%> (-3.87%)	⬇️
...ck/composite/accessevent/SequenceJsonProvider.java	0.00% <0.00%> (ø)
...n/java/net/logstash/logback/util/LogbackUtils.java	40.00% <50.00%> (ø)
...ogback/composite/AbstractSequenceJsonProvider.java	86.95% <86.95%> (ø)
...posite/loggingevent/LoggingEventJsonProviders.java	87.23% <100.00%> (+0.56%)	⬆️
...k/composite/loggingevent/SequenceJsonProvider.java	100.00% <100.00%> (ø)
...ck/appender/AbstractLogstashTcpSocketAppender.java	69.23% <0.00%> (-0.52%)	⬇️

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