Support Age header

[k1LoW]

https://httpwg.org/specs/rfc9111.html#rfc.section.4.2.3

[github-actions]

Code Metrics Report

	main (4e77272)	#28 (e9a5770)	+/-
Coverage	82.1%	83.2%	+1.2%
Code to Test Ratio	1:2.1	1:2.2	+0.1
Test Execution Time	37s	34s	-3s

Details

  |                     | main (4e77272) | #28 (e9a5770) |  +/-  |
  |---------------------|----------------|---------------|-------|
+ | Coverage            |          82.1% |         83.2% | +1.2% |
  |   Files             |              3 |             4 |    +1 |
  |   Lines             |            234 |           256 |   +22 |
+ |   Covered           |            192 |           213 |   +21 |
+ | Code to Test Ratio  |          1:2.1 |         1:2.2 |  +0.1 |
  |   Code              |            538 |           579 |   +41 |
+ |   Test              |           1119 |          1245 |  +126 |
+ | Test Execution Time |            37s |           34s |   -3s |

Code coverage of files in pull request scope (87.4% → 88.5%)

Files	Coverage	+/-
rfc9111/age.go	100.0%	+100.0%
rfc9111/shared.go	86.9%	-0.5%

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Reported by octocov