Use encoding/json as JSON decoder instead of mapstructure

[s-mang]

After research and benchmarking, I have determined that using the Go standard library's encoding/json as a JSON decoder instead of mitchellh/mapstructure will yield massive performance gains, particularly in the case where we are passing around large JSON values (benchmarks are showing ~300x speed improvement in this particular case).

In this PR, I have changed JSON decoding in our agent code to use only the standard library's encoding/json.

Users should see immediate performance improvements for most HTTP agent endpoints with a request body. Most endpoints should see a 1x-10x improvement in decoding time per request, regardless of the request body. K/V writes (Txn endpoint) should see O(100x) decoding time improvements for large Values.
Resource utilization for agent JSON decoding should also improve with this PR, ~same order of magnitude per endpoint.

Fixes #6147

[s-mang]

Here are before/after benchmarks per endpoint. These values == $before / $after. Remember that small deviations in benchmarks can't be taken too seriously. Look for the large factors.

Note the last value in particular (the original bug).

	Small JSON	All Vals 1KB	Small except 1 val is 100KB
ACLPolicyWrite	4	1	0.5
ACLToken	5	1	0.5
ACLTokenClone	5	1	0.5
ACLRoleWrite	6.666666667	1	0.5
ACLBindingRuleWrite	3	1.5	1
ACLAuthMethodWrite	3	1.666666667	1
ACLLogin	3	1.5	1
ACLUpdate	5	1.666666667	1
AgentRegisterCheck	5	1	0.5
AgentCheckUpdate	5	2	1
AgentRegisterService	3.333333333	0.5	1
AgentToken	5	2	1
AgentConnectAuthorize	10	1	1
CatalogRegister	5	1	1
CatalogDeregister	5	1.5	1
ConnectCAConfigurationSet	5	4	1
CoordinateUpdate	4	2.5	1
DiscoveryChainRead	6	1.666666667	0.5
IntentionCreate	5	1	0.5
IntentionSpecificUpdate	3.333333333	1	0.5
OperatorKeyringEndpoint	10	2	1
OperatorAutopilotConfiguration	6	3.333333333	1
PreparedQueryGeneral_Create	5	2	1
PreparedQueryGeneral_Update	10	1	1
SessionCreate	4	0.5	0.5
TxnConvertOps	5	1	100

[mkeeler]

Overall this is looking good. I left a handful of questions inline.

[s-mang]

Thanks for the review!

[mkeeler]

LGTM

[hanshasselberg]

I am late to the party, but the numbers are exciting!!

[ghost]

Hey there,

This issue has been automatically locked because it is closed and there hasn't been any activity for at least 30 days.

If you are still experiencing problems, or still have questions, feel free to open a new one 👍.