feat: Reworked Buffer and Flusher Interfaces and implementations.

[cpheps]

Description of Changes

• Reworked the buffer interface
• Rewrote memory and disk buffers
• Refactored how flusher works
• Updated operators that use buffer and flusher

Please check that the PR fulfills these requirements

• [X] Tests for the changes have been added (for bug fixes / features)
• [X] Docs have been added / updated (for bug fixes / features)
• [x] CI passes

[codecov[bot]]

Codecov Report

Merging #589 (3bcf338) into v2 (793336c) will increase coverage by 5.73%. The diff coverage is 85.13%.

@@            Coverage Diff             @@
##               v2     #589      +/-   ##
==========================================
+ Coverage   67.01%   72.73%   +5.73%     
==========================================
  Files          63       75      +12     
  Lines        4392     5593    +1201     
==========================================
+ Hits         2943     4068    +1125     
- Misses       1172     1283     +111     
+ Partials      277      242      -35     

Impacted Files	Coverage Δ
operator/buffer/buffer.go	75.86% <ø> (-1.92%)	:arrow_down:
...builtin/input/aws/cloudwatch/cloudwatch_persist.go	62.50% <ø> (-8.93%)	:arrow_down:
operator/builtin/input/azure/event_hub.go	0.00% <ø> (ø)
operator/builtin/input/azure/event_hub_config.go	90.91% <ø> (-1.09%)	:arrow_down:
operator/builtin/input/file/finder.go	100.00% <ø> (ø)
operator/builtin/input/file/reader.go	53.05% <0.00%> (-5.73%)	:arrow_down:
operator/builtin/input/http/config.go	97.03% <ø> (-0.23%)	:arrow_down:
operator/builtin/input/journald/journald.go	57.06% <ø> (-2.03%)	:arrow_down:
operator/builtin/output/googlecloud/severity.go	100.00% <ø> (+72.73%)	:arrow_up:
operator/helper/persist/cached_persister.go	100.00% <ø> (ø)
... and 88 more

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