New TS APIs - TSHttpTxnConfigParse/TSHttpTxnConfigSet

[masaori335]

Address #12292. Using MgmtConverter for list and other containers has a overhead because the per-remap configurations are stored as a string and parsed on every transactions. This PR avoids that cost by parsing the string once at config-load time by introducing new TS APIs.

New TS APIs Proposal

TS_RECORDDATATYPE_VARIANT
using TSConfigValue = std::variant<HttpStatusCodeList *>;

TSReturnCode TSHttpTxnConfigParse(TSConfigValue &dst, TSOverridableConfigKey key, const char *value, size_t length);
TSReturnCode TSHttpTxnConfigSet(TSHttpTxn txnp, TSOverridableConfigKey key, const TSConfigValue &src);

TSHttpTxnConfigParse parses given string into appropriate data structure and stores it in a TSConfigValue. It's a variant of data structures. TSHttpTxnConfigSet set the parsed TSConfigValue to the transaction to override a variable.

TSConfigValue is a std::variant for type safety and future extension.

Usage

The changes in conf_remap illustrate how to use the new APIs.

1. (Re)loading config: Parse the config string into HttpStatusCodeList * by TSHttpTxnConfigParse and store it in the TSConfigValue
2. Remap: Override the variable by TSHttpTxnConfigSet

[masaori335]

After talked with @zwoop, this is trying to combine current type specific set APIs into one TSHttpTxnConfigSet.

[masaori335]

Now, this variant captures all types in the OverridableHttpConfigParams. However, as @zwoop pointed out, every time we add a new type here, ABI compatibility is broken 🤔

[masaori335]

This is getting big change, so let me divide into 2 steps.

1). Make negative_caching_list/negative_revalidating_list overridable with MgmtConverter
2). Fix overhead of MgmtConverter by introducing new TS APIs.