Added install-capi.sh

[AkshatRai07]

A step for #519
In this PR we,

• Add install-capi.sh script to install the ekore_capi releases.

[Copilot]

Pull request overview

Adds a POSIX sh installer script for the ekore_capi release artifacts, intended to simplify installing the C/C++ interface deliverables discussed in #519.

Changes:

• Introduces crates/ekore_capi/install-capi.sh with --version, --prefix, and --target argument parsing plus platform-based target autodetection.
• Downloads and extracts the appropriate ekore_capi-<version>-<target>.tar.gz release artifact and patches the installed ekore_capi.pc.
• Checks for pkg-config/pkgconf and prints guidance/warnings about PKG_CONFIG_PATH.

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[AkshatRai07]

So we would need to release to actually see if it is working or not, otherwise it is working till this step:

$ ./crates/ekore_capi/install-capi.sh --prefix ~/.local
prefix:  '/home/akshatrai/.local'
target:  'linux-x86_64'
version: 'v0.15.4'
URL:     'https://github.com/NNPDF/eko/releases/download/v0.15.4/ekore_capi-v0.15.4-linux-x86_64.tar.gz'
curl: (22) The requested URL returned error: 404

gzip: stdin: unexpected end of file
tar: Child returned status 1
tar: Error is not recoverable: exiting now

Second, this install-capi.sh will be pushed to crates.io, you can check by running cargo package --manifest-path crates/ekore_capi/Cargo.toml --list --allow-dirty, do we need that or not?

[felixhekhorn]

Second, this install-capi.sh will be pushed to crates.io, you can check by running cargo package --manifest-path crates/ekore_capi/Cargo.toml --list --allow-dirty, do we need that or not?

we don't need it, I'd say, so we can drop it - but it would be only a single (random) text file, so no big harm I would say ...

[felixhekhorn]

@cschwan @Radonirinaunimi you may want to port some of the fixes from here to PineAPPL 😇