kci command line tool rework

The `kci` command is to the new API & Pipeline what the `kci_*` tools were for the legacy system. It needs to be clearly designed to ease adoption by the community as a tool for KernelCI core developers but also any member of the kernel ecosystem: kernel developers, test system developers etc. I should help with prototyping features by hand before automating them with services running event loops and also work hand-in-hand with the new web dashboard.

Several aspects need to be considered, in particular: feature set, ease of implementation and maintenance of the code, direct user experience, documentation, accessibility.

Here are some of the topics as part of the preparation work:
- [x] https://github.com/kernelci/kernelci-project/discussions/258
- [x] https://github.com/kernelci/kernelci-project/discussions/263
- [x] https://github.com/kernelci/kernelci-project/discussions/261

Based on the outcome of these, here's the list of tasks to complete:
- [x] Legacy code moved to `kernelci.legacy`
- [x] #2127
- [x] https://github.com/kernelci/kernelci-pipeline/pull/338
- [x] New `kci` entry point command implemented using chosen framework
- [x] #2133
- [x] TOML settings features integrated with arguments parsing and secrets
- [x] #2129
- [ ] Common options and CLI framework
- [x] #2178
- [x] #2195
- [x] #2230
- [x] #2233
- [ ] Required set of sub-commands implemented
- [ ] `kci user`
- [x] #2141
- [x] #2183
- [x] #2084
- [x] #2205
- [x] #2239
- [x] #2236
- [ ] `kci config`
- [x] #2138
- [ ] #2226
- [x] `kci job`
- [x] #2220
- [x] #2227
- [x] `kci node`
- [x] #2204
- [x] #2231
- [x] `kci event`
- [x] #2203
- [x] `kci storage`
- [x] #2161
- [x] #2134
- [ ] Documentation updated accordingly
- [ ] HTML / Markdown for online pages
- [ ] man pages and offline help

Some follow-up GitHub issues will be created for additional commands such as `kci view`, `kci bisect`, `kci email` etc.