move multiline handling from file input to helper package

[jsirianni]

Description of Changes

Before adding multiline support to TCP / UDP input, we need to pull multiline handling out of file input and into helper. This was already implemented in Open Telemetry: open-telemetry/opentelemetry-log-collection#125

• The public interface (input params) are the same, and should not break existing configurations
• This PR also adds test data for file input.
• Tested multiline behavior and it appears to behave the same as master branch

TCP and UDP multiline support will come after this PR is merged.

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Add a changelog entry (for non-trivial bug fixes / features)
• CI passes

[djaglowski]

Log Files	Logs / Second	CPU Avg (%)	CPU Avg Δ (%)	Memory Avg (MB)	Memory Avg Δ (MB)
1	1000	1.3965582	+0.120655656	127.88066	-0.74568176
1	5000	4.810407	+0.22416782	137.65207	+2.1371155
1	10000	9.793234	-0.034589767	144.12311	+0.15206909
1	50000	47.913868	-1.224453	172.28691	-2.131195
1	100000	97.34235	+2.0480347	235.52196	-4.50148
10	100	1.6380061	-0.12063444	132.55038	-1.4870605
10	500	5.827646	-0.91397285	139.96214	+0.9000397
10	1000	10.620724	-0.03459263	146.24637	-2.2285767
10	5000	52.300335	-1.3560181	180.48747	+6.2188873
10	10000	106.32825	+4.161766	230.533	+12.661377

[djaglowski]

Log Files	Logs / Second	CPU Avg (%)	CPU Avg Δ (%)	Memory Avg (MB)	Memory Avg Δ (MB)
1	1000	1.4655461	+0.18964362	129.29257	+0.66622925
1	5000	5.327698	+0.7414589	137.28825	+1.7733002
1	10000	9.793391	-0.03443241	146.1937	+2.2226562
1	50000	49.512367	+0.37404633	178.17148	+3.7533722
1	100000	90.79407	-4.500244	228.26091	-11.762527
10	100	1.793089	+0.034448504	134.91743	+0.8799896
10	500	5.8621264	-0.8794923	138.62918	-0.43292236
10	1000	11.948475	+1.2931585	148.1832	-0.29174805
10	5000	55.45649	+1.8001366	177.3168	+3.0482178
10	10000	106.58609	+4.419609	227.07475	+9.203125

[djaglowski]

Log Files	Logs / Second	CPU Avg (%)	CPU Avg Δ (%)	Memory Avg (MB)	Memory Avg Δ (MB)
1	1000	1.3276224	+0.051719904	129.84294	+1.2165985
1	5000	4.5862193	-2.002716e-05	137.37433	+1.859375
1	10000	9.6036	-0.22422409	146.79445	+2.82341
1	50000	50.15443	+1.0161095	176.19571	+1.7776031
1	100000	90.44812	-4.8461914	234.25108	-5.772354
10	100	1.7414056	-0.017234921	133.6743	-0.36314392
10	500	6.206903	-0.53471565	140.7512	+1.6891022
10	1000	10.707217	+0.051900864	144.3226	-4.1523438
10	5000	57.25976	+3.6034088	178.14116	+3.8725739
10	10000	104.77508	+2.6085968	223.46376	+5.5921326

[codecov]

Codecov Report

Merging #319 (ff98aa9) into master (34a85da) will decrease coverage by 0.05%.
The diff coverage is 68.06%.

@@            Coverage Diff             @@
##           master     #319      +/-   ##
==========================================
- Coverage   70.09%   70.04%   -0.05%     
==========================================
  Files         119      121       +2     
  Lines        6330     6369      +39     
==========================================
+ Hits         4437     4461      +24     
- Misses       1422     1443      +21     
+ Partials      471      465       -6     

Impacted Files	Coverage Δ
operator/builtin/input/file/file.go	75.86% <ø> (+2.07%)	⬆️
operator/helper/encoding.go	53.85% <53.85%> (ø)
operator/helper/multiline.go	80.77% <62.07%> (ø)
operator/builtin/input/file/config.go	78.12% <100.00%> (-2.52%)	⬇️
operator/builtin/input/file/reader.go	67.86% <100.00%> (ø)
operator/helper/operatortest/operatortest.go	100.00% <100.00%> (ø)
operator/builtin/output/otlp/otlp.go	61.73% <0.00%> (-3.70%)	⬇️
operator/builtin/output/forward/forward.go	55.07% <0.00%> (-1.45%)	⬇️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 34a85da...ff98aa9. Read the comment docs.

[djaglowski]

Log Files	Logs / Second	CPU Avg (%)	CPU Avg Δ (%)	Memory Avg (MB)	Memory Avg Δ (MB)
1	1000	1.431056	+1.9550323e-05	127.299706	-0.25862122
1	5000	5.0689917	+0.051639557	135.37392	-2.1064148
1	10000	9.81047	+0.51739407	142.99205	-2.3133087
1	50000	47.25964	-3.7589684	178.29579	-1.5126648
1	100000	99.82889	+8.500259	226.61638	-0.6441345
10	100	1.7931266	-0.12071848	134.90019	+3.1914062
10	500	5.8793793	-0.017116547	139.6452	+0.020339966
10	1000	11.844902	+0.15513134	146.40921	-1.7320862
10	5000	50.99617	-4.918644	177.29593	-1.744339
10	10000	105.118645	+6.971611	221.59496	+0.94369507