Add start_at parameter to journald input

[camdencheek]

Description of Changes

Adds a configurable start_at parameter to the journald input plugin, which previously could only start at the end.

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 Average (%)	Memory Average (MB)
1	1000	2.499999	26.652075
1	5000	6.344973	35.1545
1	10000	10.0518875	45.081223
1	50000	39.25988	125.10251
1	100000	70.191216	188.9639
10	100	4.172506	27.781115
10	500	8.396701	34.502964
10	1000	12.638191	45.482624
10	5000	46.8122	91.56048
10	10000	83.538315	177.45433

[djaglowski]

Log Files	Logs / Second	CPU Average (%)	Memory Average (MB)
1	1000	2.8966355	26.372307
1	5000	6.1725106	35.992188
1	10000	10.5346775	43.298626
1	50000	38.380413	117.3482
1	100000	70.96684	167.3618
10	100	4.7414865	28.177801
10	500	8.086326	36.969826
10	1000	12.155414	42.08944
10	5000	46.22516	106.2073
10	10000	79.84669	165.96188

[djaglowski]

Log Files	Logs / Second	CPU Average (%)	Memory Average (MB)
1	1000	2.7931268	27.949488
1	5000	6.086308	34.628635
1	10000	11.189987	45.18063
1	50000	39.310616	106.36826
1	100000	70.013664	162.5427
10	100	4.569045	28.323006
10	500	7.4656262	33.728718
10	1000	14.017502	44.35493
10	5000	45.380375	84.871635
10	10000	82.16582	178.37271

[codecov]

Codecov Report

Merging #55 into master will decrease coverage by 0.08%.
The diff coverage is 42.86%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
- Coverage   74.94%   74.86%   -0.08%     
==========================================
  Files          63       63              
  Lines        3835     3842       +7     
==========================================
+ Hits         2874     2876       +2     
- Misses        718      722       +4     
- Partials      243      244       +1     

Impacted Files	Coverage Δ
operator/builtin/input/journald.go	54.63% <42.86%> (-0.82%)	⬇️
operator/builtin/input/tcp.go	84.00% <0.00%> (-1.33%)	⬇️

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