[feat] Add EsgfQuery class to wrap ESGF search RESTful APIs

[hongyuanjia]

Pull request overview

• Fixes Use utils:URLencode() #62
• Fixes Refactor esgf_query() to enable a step-by-step query builder #63
• Fixes Use offset for query result pagination #65

This is a big PR with a lot of refactorings and new features.

• Follow the tidyverse style, the name of environment to store the internal state has been changed from EPWSHIFTR_ENV to this.
• A new R6 class EsgfQuery has been added to wrap the ESGF search RESTful APIs in a more flexible way. This allows to build a query in a step by step, e.g. query_esgf()$project("CMIP6")$activity_id("ScenarioMIP")$experiment_id(!"ssp126")$collect(). This solves Refactor esgf_query() to enable a step-by-step query builder #63 and Use offset for query result pagination #65.
• TBH, the function name of the EsgfQuery$new() should be esgf_query(). Unfortunately, this name has been taken.
• esgf_query() has also utilized the new query URL encoding function. This solves Use utils:URLencode() #62. Note that utils::URLencode() is not used, since it also encodes other characters like !, +, etc.
• Complete the test migration to use {testthat} 3rd edition.

[codecov-commenter]

Codecov Report

Merging #73 (0d0ff9c) into master (ce7dadb) will decrease coverage by 0.02%.
The diff coverage is 99.71%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
- Coverage   99.82%   99.80%   -0.03%     
==========================================
  Files           7        8       +1     
  Lines        1678     2005     +327     
==========================================
+ Hits         1675     2001     +326     
- Misses          3        4       +1     

Impacted Files	Coverage Δ
R/morph.R	100.00% <ø> (ø)
R/utils.R	96.87% <92.30%> (-3.13%)	⬇️
R/dict.R	99.75% <100.00%> (+<0.01%)	⬆️
R/esgf.R	100.00% <100.00%> (ø)
R/netcdf.R	99.48% <100.00%> (ø)
R/query.R	100.00% <100.00%> (ø)

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