Added inject function

[enrichman]

Fixes #142

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This PR adds an inject function that will override the original value if the percentage matches.

A 1 value means "always inject", while the 0 is "never".

A 0.5 will match with a probability of 50%.

-> % jr template run --embedded '{{city | inject 0.5 "<FAIL>" }}'
2024/07/16 22:55:49 JR configuration not found
<FAIL>

Elapsed time: 0s
Data Generated (Objects): 1
Data Generated (bytes): 6
Throughput (bytes per second):        49

-> % jr template run --embedded '{{city | inject 0.5 "<FAIL>" }}'
2024/07/16 22:55:51 JR configuration not found
Houston

Elapsed time: 0s
Data Generated (Objects): 1
Data Generated (bytes): 7
Throughput (bytes per second):        58